This invention relates to gaseous fuel powered internal combustion engines, and more particularly, it relates to a method of operating an internal combustion engine using a hydrogen-rich fuel to produce exhaust gas having near-zero NOx.
Internal combustion engines are commonly used in motor vehicles as well as for other purposes such as power generation. In the last few decades, federal and state regulatory agencies have encouraged engine manufacturers to produce cleaner burning engines. Reduced vehicle emissions have been achieved largely due to modifications in the engine design, with some modifications to the traditional hydrocarbon fuels. More recently, the federal and state agencies have been encouraging the use of alternative fuels, particularly, gaseous fuels such as hydrogen, natural gas, propane and butane. In general, such gaseous fuels tend to burn cleaner than conventional hydrocarbon fuels. Hydrogen has generally been considered a particularly clean fuel since theoretically, its only combustion product is water vapor. However, the combustion of such fuels can still tend to produce a significant amount of emissions of oxides of nitrogen (NOx). In particular, because hydrogen has a high flame temperature, and because NOx tend to form at high temperatures hydrogen-rich fuels tend to burn hotter, producing high levels of NOx emissions.
In order to reduce NOx emissions, some engine manufacturers have designed engines in which additional gases are introduced into the fuel stream to dilute the fuel charged into the combustion chamber. Such dilution tends to reduce the flame temperature, thereby reducing NOx emissions. Common methods of diluting the charge include the addition of excess air to the air-fuel mixture charged to the combustion chamber to operate under xe2x80x9clean-burnxe2x80x9d conditions, or the use of exhaust gas recirculation (EGR). The term xe2x80x9cexcess airxe2x80x9d is intended to mean an amount of air in excess of the stoichiometric amount necessary to support complete combustion of the fuel. The use of charge dilution results in an increase in the heat capacity of the gases used in the combustion process. The increase in heat capacity in turn reduces the peak temperature of the combustion process. By sufficiently reducing the peak temperature, NOx emissions can be significantly reduced.
One problem with operating an engine at high charge dilution is that for some fuels, the charge mixture can become too lean to support complete combustion, resulting in a xe2x80x9cmisfirexe2x80x9d condition. Misfire not only results in a severe drop in engine efficiency, but also results in high emissions of unburned hydrocarbons in the engine exhaust. Consequently, while high levels of charge dilution are desired to produce low NOx emissions, near-zero NOx emissions are difficult to achieve because of misfire.
Some engine designers have overcome such limits on the levels of charge dilution possible by adding a fuel such as hydrogen to the fuel charge. Hydrogen has range of flammability that is wider than the flammability ranges of most hydrocarbons, and therefore, the addition of hydrogen permits an engine to operate at high levels of charge dilution without causing engine misfire.
Another factor that must be considered when designing an engine to operate at high levels of charge dilution is that good mixing of the mixture charged to the combustion chamber is critical for both efficient combustion and low emissions. Without good mixing, xe2x80x9chot spotsxe2x80x9d can develop in certain areas within the combustion chamber causing an increase in NOx emissions. The most common means of achieving good mixing is through engine design whereby high levels of internal angular momentum are imparted to the charge mixture as it is introduced into the combustion chamber. Such high levels of angular momentum generally promote turbulent mixing of the air-fuel mixture, thereby promoting faster combustion flame speeds in highly diluted mixtures. For liquid fuels such as gasoline, the high angular momentum further promotes the vaporization of the fuel.
Currently there are two commonly used methods for generating high angular momentum in the charge mixture. The first method is to configure the engine intake ports and/or the intake valves relative to the engine cylinder so that the charge mixture circulates within the cylinder. The object of this method is to sustain an overall circular motion within the cylinder until the piston reaches the bottom of its travel during the intake stroke. As the cylinder moves upward in the cylinder on the compression stroke, the remaining circular motion of the charge mixture is amplified by the decreasing volume in the cylinder. The resulting circular or vortex motion of the charge mixture at the point of initial combustion can be quite high causing excellent mixing for a charge mixture composed of hydrocarbons, air and recycle gases. In one type of intake port design, the intake port has a shallow angle of flow relative to the valve face in order to generate a significant flow velocity in a plane perpendicular to the cylinder centerline. According to another intake port design, a helical port is used to impart a vortex motion to the charge before it reaches the intake valve. Intake valves can also be designed with shrouds to cause unequal flow about the intake valve to generate a vortex flow within the engine cylinder. Furthermore, multiple intake valves can be used whereby the valves introduce the charge mixture into the combustion chamber in different directions to promote vortex flow.
The second method for generating high angular momentum in the charge mixture is through combustion chamber design. Rapidly moving circular vortices in the charge mixture can be promoted by designing the shape of the combustion chamber to cause high angular momentum, especially during the compression cycle. This is commonly achieved by the piston crown design. Commonly used piston crowns designed to promote vortex flow include the xe2x80x9cbathtubxe2x80x9d design, xe2x80x9cbowl-in-pistonxe2x80x9d design, xe2x80x9cnebulaxe2x80x9d design, xe2x80x9creentrantxe2x80x9d design, and the xe2x80x9cTGxe2x80x9d design.
Cylinder head designs can also be used to modify the shape of the combustion chamber, thereby contributing to high angular momentum within the cylinder. For example, cylinder heads can be designed with shrouds similar to intake valve shrouds, or with a xe2x80x9cchamber-in-headxe2x80x9d design, either of which will cause unequal flow about the intake valve, resulting in the generation of vortex flow within the combustion chamber. Any of these designs, or combinations of the designs can be used to promote vortex flow within the combustion chamber.
Regardless of whether either of these two methods or some other method is used to achieve high angular momentum within the combustion chamber, two types of angular momentum are generally recognized. The first is where the circular motion within the combustion chamber is generally about an axis defined by the centerline of the engine cylinder. This type of angular momentum is called xe2x80x9cswirl.xe2x80x9d The second is where the circular motion within the combustion chamber is generally about an axis perpendicular to the axis defined by the centerline of the engine cylinder. This type of angular momentum is called xe2x80x9ctumble.xe2x80x9d
For lean-burn engines operating on natural gas and other gaseous hydrocarbons, maintaining good mixing of the charge mixture such as by using a combustion chamber with high levels of swirl and tumble is critical for preventing incomplete combustion and maintaining low NOx emissions.
While commonly used methods of promoting swirl and tumble within a combustion chamber have proven effective in producing low-emission engines that run on hydrocarbon fuels, for an engine running on a hydrogen-rich fuel, it has been discovered by the inventor that causing high levels of swirl and tumble can lead to higher emissions and poorer engine performance than would otherwise be expected. It is theorized that where a charge mixture containing a hydrogen-rich fuel is subjected to high levels of angular momentum, rather than promoting good mixing of the charge mixture, the resulting centrifugal forces actually cause separation of the charge mixture.
For a lean-burn engine operating on a hydrogen-rich fuel, the major constituents of the charge mixture will be hydrogen, methane, nitrogen, oxygen, carbon dioxide and water vapor. Given that the molecular weight of hydrogen is approximately 2, methane is approximately 16, nitrogen is approximately 28, oxygen is approximately 32, carbon dioxide is approximately 44 and water is approximately 18, hydrogen is by far the lightest constituent in the charge mixture. Consequently, it is believed that any angular momentum of the charge mixture will produce a centrifuge effect by which the heavier components are pushed to the outer edge of the vortex which generally corresponds with the walls of the combustion chamber while the hydrogen remains concentrated near the center of the vortex, typically at the center of the combustion chamber near the spark plug. The resulting areas of high concentration of hydrogen result in localized hot spots. Conversely, in those areas where hydrogen concentration is low, complete combustion may not be possible. The net result is higher emissions, and in particular, higher NOx emissions, and often higher hydrocarbon than would be achieved if the hydrogen were more evenly distributed throughout the combustion chamber.
The present invention is directed to a method of operating an internal combustion engine under high charge dilution conditions using a hydrogen-rich fuel. The high charge dilution conditions are attained by diluting the hydrogen-rich fuel with gases such as excess air or recycled exhaust gas. The mixture to be charged to the combustion chamber is thoroughly mixed before it is introduced into the combustion chamber while the combustion chamber maintains a substantially quiescent state such that the charge mixture is introduced with an angular momentum lower than that of a typical low emission engine. The result is an internal combustion engine with both high efficiency and low emissions, particularly, low NOx emissions.